(1) Field of the Invention
The invention relates to a method of manufacturing a silicon condenser microphone, and more particularly, to a method of manufacturing a high performance silicon condenser microphone using a silicon micro-machining process.
(2) Description of the Prior Art
Silicon condenser microphones have long been an attractive research and development subject. Various microphone designs have been invented and conceptualized by using silicon micro-machining technology. Despite various structural configurations and materials, the silicon condenser microphone consists of four basic elements: a movable compliant diaphragm, a rigid and fixed backplate (which together form a variable air gap capacitor), a voltage bias source, and a pre-amplifier. These four elements fundamentally determine the performance of the condenser microphone. In pursuit of high performance; i.e., high sensitivity, low bias, low noise, and wide frequency range, the key design considerations are to have a large size of diaphragm and a large air gap. The former will help increase sensitivity as well as lower electrical noise, and the later will help reduce acoustic noise of the microphone. However, the large diaphragm requires a large span of anchored supports and correspondingly a large backplate. Also, a large air gap requires a thick sacrificial layer. These present major difficulties in/silicon micro-machining processes. Due to constraints of material choices and intrinsic stress issues in silicon micro-machining, the silicon microphones reported so far have not achieved sensitivity of more than 20 mV/Pa.
Miniaturized silicon microphones have been extensively developed for over sixteen years, since the first silicon piezoelectric microphone reported by Royer in 1983. In 1984, Hohm reported the first silicon electret-type microphone, made with a metallized polymer diaphragm and silicon backplate. And two years later, he reported the first silicon condenser microphone made entirely by silicon micro-machining technology. Since then a number of researchers have developed and published reports on miniaturized silicon condenser microphones of various structures and performance.
Some of these reports include the following:    1) D. Hohm and R. Gerhard-Multhaupt, “Silicon-dioxide electret transducer”, J. Acoust. Soc. Am., Vol. 75, 1984, pp. 1297–1298.    2). D. Hohm and G. Hess, “A Subminiature condenser microphone with silicon nitride membrane and silicon backplate”, J. Acoust. Soc. Am., Vol. 85, 1989, pp. 476–480.    3) Murphy, P. et al., “Subminiature silicon integrated electret capacitor microphone”, IEEE Trans. Electr. Ins., Vol. 24, 1989, pp. 495–498.    4) Bergqvist, J. et al., “A new condenser microphone in silicon”, Sensors and Actuators, Vol. A21–23, 1990, pp. 123–125.    5) Kuhnel, W. et al., “A Silicon condenser microphone with structured backplate and silicon nitride membrane,” Sensors and Actuators, Vol. 30, 1991, pp. 251–258.    6) Scheeper, P. R. et al., “Fabrication of silicon condenser microphones using single wafer technology”, Journal of Microelectromechanical Systems, Vol. 1, No. 3, 1992, pp. 147–154.    7) Scheeper, P. R. et al., “A Review of Silicon Microphones”, Sensors and Actuators A, Vol. 44, Jul. 1994, pp. 1–11.    8) Bergqvist, J. et al., “A Silicon Microphone using bond and etch-back technology”, Sensors and Actuators A, vol. 45, 1994, pp. 115–124.    9) Zou, Quanbo et al., “Theoretical and experimental studies of single-chip-processed miniature silicon condenser microphone with corrugated diaphragm”, Sensors and Actuators A, Vol. 63, 1997, pp. 209–215.    10) Brauer, M. et al., “Silicon microphone based on surface and bulk micromachining”, Journal of Micromech. Microeng., Vol. 11, 2001, pp 319–322.    11) Bergqvist, J. and V. Rudolf, “A silicon condenser microphone with a highly perforated backplate”, Transducer 91, pp. 266–269.
U.S. Pat. No. 5,870,482 to Loeppert et al reveals a silicon microphone. U.S. Pat. No. 5,490,220 to Loeppert shows a condenser and microphone device. U.S. Patent Application Publication 2002/0067663 to Loeppert et al shows a miniature acoustic transducer. U.S. Pat. No. 6,088,463 to Rombach et al teaches a silicon condenser microphone process. U.S. Pat. No. 5,677,965 to Moret et al shows a capacitive transducer. U.S. Pat. Nos. 5,146,435 and 5,452,268 to Bernstein disclose acoustic transducers. U.S. Pat. No. 4,993,072 to Murphy reveals a shielded electret transducer.
However, none of the silicon condenser microphones mentioned above has been reported to achieve sensitivity above 20 mV/Pa. In terms of conventional condenser microphones (i.e. non-silicon), very few products can have sensitivity as high as 100 mV/Pa. For example, Bruel & Kjoer, Denmark (B&K) has only one microphone available with this high sensitivity (B&K 4179, 1-inch diameter). Its dynamic range is about 140 dB (200 Pa) and frequency range is 5–7 kHz. However, this microphone must be fit onto a bulky pre-amplifier and requires a polarization voltage of 200 V.